The "Sunny Border Blue" variety of Veronica grows about 2' tall in my garden.  It blooms all summer long starting in late June.  The clumps are supposed grow vigorously, but I have found this not to be the case.  It has been a struggle just to keep them alive.  In 2003, I divided what was left of them into as many pieces as possible, in an attempt to rejuvenate them.  They seem to be doing better now.  Sunny Border Blue is very good butterfly plant.